1. Field of the Invention
The present invention relates to a method and apparatus for tracking error detection and more particularly, to an improved method and apparatus for tracking error detection in which a phase locked loop (PLL) is introduced into a conventional differential phase detection tracking error (DPD TE) method to increase the accuracy of tracking error detection.
2. Description of the Related Art
In a conventional DPD TE method, phase differences are generated on the edges of pits or marks of an optical disk. The length of pits or marks recorded on an optical disk lies in various ranges. For example, in the case of digital versatile disk-ROM (DVD-ROM), a length ranges from 3T to 14T where T is the duration of a channel clock of the disk. If there are a lot of pits or marks having a short length, phase difference detection can be performed many times, thereby enhancing the reliability of a tracking error signal derived therefrom. Conversely, if there are more pits or marks having a long length, the number of times phase difference detection may be done is reduced, thereby degrading the reliability of a tracking error signal. Further, a spectrum component, according to a modulation method of signal recorded on a disk, is closely related to outputs of AC+ and BD+, and a low-frequency component of the spectrum acts on noise with regard to a tracking error signal which is used for following and determining the position of a tracking center.
According to a conventional DPD TE method, phase difference detection is supposed to be made from pits or marks at one time, so that the gain and characteristics of a detected signal deteriorate if the signal of pits or marks is adversely affected by defects or the like. In addition, as the track density of an optical disk increases, the magnitude and gain of a tracking error signal according to the conventional DPD TE method decrease. Thus, the conventional DPD TE method has a disadvantage in that it is difficult to precisely control tracking in a high-density track structure. Referring to FIG. 1, the configuration of a tracking error detecting apparatus according to a conventional differential phase detection tracking error (DPD TE) method is shown. The apparatus shown in FIG. 1 includes a four-section optical detection unit 102, a matrix circuit 104, high-pass filters (HPFs) 106a and 106b, comparators 108a and 108b, a phase comparator 110, and a low-pass filter (LPF) 112. The apparatus detects a phase difference between the signals output from the four-section optical detection unit 102 to determine the position of a laser spot. If the laser spot deviates from a track center, then a time delay or a phase difference between A+C and B+D signals results. Thus, a tracking error signal is generated by detecting the time delay between those signals.
Specifically, the matrix circuit 104 adds optical detection signals A and B, and C and D, which are positioned along a diagonal line among the outputs (A, B, C and D) of the four-section optical detection unit 102, and outputs AC1 and BD1 from A+C and B+D, respectively. The HPFs 106a and 106b reinforce the high-frequency components of AC1 and BD1 provided from the matrix circuit 104, differentiate AC1 and BD1, and output the results, i.e., AC2 and BD2 to the comparators 108a and 108b. The comparators 108a and 108b binarize each of AC2 and BD2 provided from the HPFs 106a and 106b, compare AC2 and BD2 with a predetermined level (a ground level in FIG. 1) to output the results, i.e., AC3 and BD3 to the phase comparator 110.
The phase comparator 110 detects a phase difference between AC3 and BD3 provided from the comparators 108a and 108b, compares the phases of AC3 and BD3 to output the results, i.e., AC+ and BD+ to the LPF 112. In this case, AC+ is a phase difference signal generated when AC3 leads BD3 in phase, while BD+ is a phase difference signal generated when BD3 leads AC3 in phase. The LPF 112 filters AC+ and BD+ input from the phase comparator 110 and outputs the result as a tracking error signal.
FIGS. 2A-2D are waveform diagrams illustrating operation of the apparatus shown in FIG. 1. FIGS. 2A-2D show the case in which AC3 leads BD3 in phase. The wave forms of AC3, BD3, AC+ and BD+ signals are illustrated sequentially from FIG. 2A to FIG. 2D. As shown in FIGS. 2A-2D, it can be found that if a laser spot deviates by a predetermined amount, there exists a phase difference between AC3 and BD3, shown in FIG. 2A and FIG. 2B, respectively, which is in turn reflected into AC+ and BD+, shown in FIG. 2C and FIG. 2D, respectively. If AC3 leads BD3 in phase, a tracking error signal is greater than a predetermined central value, but in the opposite case, it is less than the predetermined central value. The degree to which a tracking error signal deviates from the central value corresponds to the distance by which the laser spot is departed from the track center.
The phase comparator 110 of the apparatus shown in FIG. 1 detects a phase difference at a rising or falling edge of AC3 and BD3. The rising or falling edges of AC3 and BD3 correspond to the edges of pits or marks recorded on an optical disk. In other words, the apparatus shown in FIG. 1 detects a phase difference once on every edge of pits and marks recorded on an optical disk. Thus, as the number of pits or marks increases, the reliability of a tracking error signal increases, and as the number of pits or marks decreases, the reliability of the signal decreases. If pits or marks are affected by defects of an optical disk or other factors, the gain and characteristics of a tracking error signal become worse. A spectrum component according to a recording modulation method is closely connected with AC+ and BD+, and especially a low-frequency component of the spectrum works on noise with regard to a tracking error signal. Further, in the case of a tracking error signal according to the DPD TE method, the magnitude and gain are reduced as track density is increased, which makes the accurate control of tracking in a high track- density structure difficult.